This invention pertains to mounting of retractable propulsive and station-keeping marine thrusters in a vessel hull. More particularly, it pertains to such mountings which provide deployed, retracted, and elevated (service or maintenance) positions of a thruster and its equipment canister relative to a vessel hull.
The worldwide search for oil and gas is extending farther and farther offshore from land. That search includes the drilling of exploratory and of production wells in the sea floor at locations of greater and greater depth. Wells now are being drilled in water depths which are sufficiently great that it is impractical, sometimes impossible, to use mooring systems to hold a floating drilling facility in place on the water surface over the well location.
Drillships (i.e., vessels of generally conventional ship form, overall hull configuration) are a common type of floating drilling facility and are preferred over other types of facilities for the drilling of wells in great water depths. It is known to equip drillships with devices known as thrusters for maneuvering and for station-keeping of the vessel. Thrusters include propellers which are operated to create thrust forces which are applied to the vessel for movement of the vessel in desired directions. In a tunnel thruster, the propeller is located in a tunnel which extends transversely through the vessel below its waterline, usually near the bow or the stern of the vessel. Tunnel thrusters are used in combination with the conventional fixed axis propulsive propellers at the stem of the vessel to adjust and to maintain the heading and the position of the vessel over a well site on the sea floor. Retractable and steerable thrusters also are known in the context of drillships and other floating drilling facilities. Whereas tunnel thrusters apply thrust reaction forces to a vessel only in one or the other of two opposite directions transversely of the vessel hull, steerable thrusters apply thrust reaction forces in any desired horizontal direction relative to the hull. For that reason, steerable thrusters are increasingly preferred for station keeping of deep water drillships.
Drillships commonly are owned by firms separate from the firms (oil companies) which have rights to drill subsea wells. Drillships, therefore, are leased or chartered by their owners and operators to oil companies. The daily lease or charter fees for drillships are called day rates and they are increasingly substantial. Therefore, it is very important to the oil companies which pay day rates that a drillship be effectively useable as much as possible in well drilling operations during the course of a lease or charter. That means that it is very important that a modern drillship be able to maintain its position over a submerged well site through a wide range of sea and weather conditions. Conditions and events which require a thruster to be shut down are to be avoided or minimized.
Thrusters, of whichever kind, are the most significant source of vessel downtime, often requiring shipyard and dry dock time to repair. Weather related up time is directly related to, among other things, the amount of power a vessel is able to put into its station keeping system. Thus, the unavailability of a thruster in a deep water drillship""s station keeping system meaningfully reduces the ability of the vessel to support drilling operations as weather conditions become more severe within the design range of weather conditions.
Thruster seal arrangements have not changed significantly over the years. When shaft seals begin to leak, there are environmental as well as mechanical considerations that must be addressed. Generally, when a shaft seal begins to leak, the thruster is shut down to minimize any possible impact to the environment, and to prevent any potential mechanical damage to the thrusters due to loss of lubricating oil. Based on standard configurations for azimuthing (steerable) thrusters, repair of a leaking shaft seal requires the vessel to be moved off of its desired location and into sheltered waters for the keel haul removal of the thruster. In the case of a tunnel thruster, the repair requires extensive diver work or, worse, the dry docking of the vessel.
It will be seen, therefore, that efficient and economical operation of a modem deep water drillship which incorporates thrusters into its dynamic positioning (station keeping) system long has presented a need for innovative structural arrangements and procedures which permit a thruster to be maintained and repaired quickly and safely without moving the vessel from its desired operational location. This invention meaningfully addresses that need in the context of azimuthing (steerable) thrusters.
This invention beneficially addresses the need noted above by providing structures and procedures which enable a steerable thruster to be raised in a vessel hull from a deployed position of the thruster propeller below the hull, through a retracted position within the hull, to an elevated and dry maintenance, service and repair position. In the maintenance, service and repair position, all components of the thruster, notably its propeller and adjacent gear drive mechanism, are located above the waterline at which the hull floats. The thruster assembly preferably is movable vertically in a cooperating trunk passage in the hull and is moved in the trunk by a drive mechanism coupled between the thruster and the hull structure. As a consequence, the thruster is conveniently, quickly and safely repairable aboard the vessel. Practice of the invention reduces the duration of downtime of a thruster and maximizes the ability of the vessel to maintain station over a desired subsea location through the design range of weather conditions.